(CNN) -- David Ferrer booked his place in the semifinals of the ATP World Tour Finals after stunning world number one Novak Djokovic in straight sets at London's O2 Arena on Wednesday.
Ferrer opened the tournament with victory over fourth seed Andy Murray, and the world number five followed up that success with a superb 6-3 6-1 victory, only Djokovic's fifth defeat of the season in all tournaments.
Djokovic has been struggling with back and shoulder injuries in recent weeks, but refused to offer any excuses after his defeat.
The 24-year-old Serb told reporters: "I have no words to explain it. It was the worst match I have played all year but all credit to David. He played really well."
Ferrer added: "I think maybe he was a little tired but I played very well and didn't make any mistakes.
"It is important for me to play well here. Last year I didn't win and now I have won twice, so I am very happy."
Ferrer becomes the first player to qualify from Group A, while Djokovic could still go through if he beats compatriot Janko Tipsarevic in his remaining match on Friday.
Tipsarevic, the replacement for the injured Murray -- who was forced to quit the tournament on Tuesday -- was beaten 2-6 6-3 7-6 by Tomas Berdych in the earlier group match.
Berdych will now need to defeat Ferrer in his final tie to qualify, and hope that Djokovic slips up against Tipsarevic.
Meanwhile, Ferrer's victory ensured he joins defending champion Roger Federer -- who has qualified from Group B -- in the last four of the competition.